Bogen CHA-10 help

[blvd]

Hi,
I have a quick question about a Bogen CHA-10 amp I picked up recently.

For this one I took out the Phono input and changed the Mic input to a 1/4" input for guitar.
Well it turns out this amp has a lot of gain for its size and I'd like to add an input back before the 6AU6 but without a volume pot following it. I'd like to add the pot before the 6L6GB, that way I could still control the volume of the second channel. It looks like I could just put a pot in at R16 with the wiper going into the 6L6, right? Would a 500K pot do it?

What would the correct way to wire the input for the second channel be? Could the signal just go through a 470K resistor right into the 6AU6?

Thanks for your help!

[olive2sing]

I'd like to add an input back before the 6AU6 but without a volume pot following it.

That Phono input is a line level input, and may not have enough gain for a guitar. If you keep the Mic pre in the circuit Use the 470K resistor. That is where the 2 channels are mixed together. First I would plug a guitar into the phono in jack and see if it has enough gain with the vol. all the way up.

It looks like I could just put a pot in at R16 with the wiper going into the 6L6, right? Would a 500K pot do it?

I think you're on the right track there(But I'm not one of the experts here, perhaps one of the experts will chime in here)

[dorian]

blvd -- My modified schematic shows what I did to my own CHA-10. Mainly I used the old phono pot for a master volume control before the 6L6.

You are right that this amp has more gain than you need for a clean guitar sound, but I really like the types of overdrive distortion I can get now. With a P90 pickup I can turn up the Microphone (now Gain) control to overdrive the 6AU6, but then turn down the Phono (now Volume) to keep a reasonable output. I wasn't sure what to expect in the tone from an overdriven 6AU6, but I like it. I also use an attenuator on the amp output so I can overdrive the 6L6 at home. The amp does not get into metal territory, but for my more classic rock tones it is a lot of fun. These modifications are rather recent, so I haven't tried playing it with other musicians.

I also removed the "brilliant" setting of the tone control, which I never liked, and this cuts out the negative feedback.

I have to admit that I don't really understand the idea behind putting the tone control in the negative feedback loop. I hope someone on this forum might explain it. Another mystery is the 1500 mmfd capacitor C13. Is it for suppressing a parasitic oscillation?

[olive2sing]

Looks good! The main thing is you like the sound. Keep reading this forum and you will learn more. Then you will start to see things you might want to try.

[dorian]

Here's an update, and a question.

I experimented with different 6AU6 tubes for preamp distortion. The different tubes had real differences, and I am pretty happy with the Raytheon tube in it now. It breaks up gradually, and it's not so fizzy as many high gain amps with overdriven 12AX7 stages.

A few weeks ago I upgraded the output transformer. The new one is rated at 20 watts, and is way heavier than the one it replaced. It gives considerably better bass response. On the other hand, the tone controls, which are in the feedback loop, are now rather unpredictable and depend on the gain settings.

After installing the output transformer, the amp developed some peculiar problems that came from the volume potentiometers. The carbon resistive layer delaminated from the phenolic backing. Perhaps the vibrations of the chassis drilling accelerated their failure. They weren't hard to replace, of course, but I've never seen this before. The amp might be more than 50 years old, though.

I had a chance to try out the amp today in a band situation with a drummer, bassist, and vocals. The speaker box was pretty efficient, and the Challenger was plenty loud enough. It had a nice growl from the output stage distortion. The only issue was that the amp seemed a bit dark. Even my Stratocaster sounded dark, which is unusual. A little equalization was helpful and made everything acceptable, but I am now thinking about how to revoice the amp a bit. From the schematic I don't see any easy component swaps to do this. Does anyone have a suggestion for giving it a bit of a boost above, say 1 kHz?

[olive2sing]

I would try changing C6, or C12, to a smaller size. Perhaps .01 instead of .033. But I'm no expert some one else might have a better idea.

[dorian]

olive2sing -- That seems a pretty fairly straightforward thing to do, and I plan to try it with C6. Thanks. I think the characteristic pass frequency of C6 is set with the 500 K pot, so this would be 10 Hz in the original design. I might experiment with a much smaller value, maybe 0.002 mfd or so.

I spent a bit of time changing the circuit since my last posting. The circuit attached with this post shows these changes:

1. I removed all the global negative feedback.

2. I removed the screen bypass cap for the 6AU6.

3. I removed the big electrolytic cap that did the cathode resistor bypass for the 6AU6, and replaced it with a much smaller one for full bypass, and a larger cap in series with a 3.3 K resistor for some midrange boost.

4. I found a tone control circuit on the internet that I put between the 6AU6 and the 6L6. It is switchable with a DPDT switch on the tone control pot. Having a single knob, it fit in the front panel where the old tone switch was located.

I heard an improvement in clarity with changes 2 and 3 above. Change 3 helped reduce the excessive bass, and gave the amp a nice voice. These changes also reduced the gain, but I can still overdrive the 6L6 when the tone control is bypassed.

Unfortunately, the tone control circuit really cuts the volume a lot. It is not possible to overdrive the 6L6 when it is in use. I haven't checked with a meter, but I don't think that the 6L6 can get to its full 10 watts when the tone control is in use, even when the 6AU6 is overdriven. It controls the tone nicely, though, and it is excellent for bedroom practice. Fortunately, the amp has a nice voice without the tone control in the circuit, so I look forward to cranking it without an attenuator when I play with my bandmates this weekend.

To get more signal past the tone control, I am thinking about changing the cathode bias, screen, and plate resistors of the 6AU6 to lower its output impedance, so perhaps the tone control will cause less of a cut in volume. Maybe I'll also reverse change 2 above. I'm not too optimistic that this will make enough of a difference to get the 6L6 to overdrive with the tone control in the circuit, though.

[dorian]

I spent time over the past year tweaking the circuit of my Challenger CHA-10, and playing it at low volume at home and higher volume at a studio with other musicians. Finally I am happy with it to the point where I don't see any need to modify it further. The schematic is attached.

One use for my revoiced amp is useful for its cleans, with the gain down low and the VVR up high. I often use its cleans for practice and for songs where I want an acoustic-like tone.

The preamp breakup of the 6AU6 pentode is unique. It is not a classic rock sound, but a more modern high-gain metal one. Maybe darker and coarser. What is interesting is how quickly the breakup occurs. I can set the gain and input level so that the soft notes are pretty clean, but get a lot of distortion when I dig in.

There are now three volume-like controls on the amp. The gain, the VVR, and the master volume. I often balance the gain against the VVR (which, after experimenting with the output tube, I set to control the plate voltages of all tubes). The lower settings of the VVR give the amp a sort of smoothness, with less articulation. The very lowest settings on the VVR cause a slow oscillation of some sort, but this is at a B+ voltage of less than 50 V.

My main expenses were:
1. new output transformer (This is necessary. The original OT is wimpy, and the PA circuit relies on a lot of negative feedback.)
2. Hall VVR
3. new pots for volume and tone (but I had these around anyhow)
4. all capacitors
5. some resistors and connectors

It all came to a something around $140. My only regret is that I did not have this schematic at the beginning. The wiring in the chassis, which was not very neat to begin with, is now a complete mess. I have had some thoughts about taking everything out and redoing it all, but the amp works fine and doesn't have noise or hum problems. I'll probably leave it alone unless something breaks.

[olive2sing]

Hey Dorian, that sounds great. I like the way you've made it versatile, yet not too complex. And if it sounds good it's worth all the tweaking. I may steal some of your ideas.

"The wiring in the chassis, which was not very neat to begin with, is now a complete mess." I can relate, if it sounds good with no noise issues, don't loose any sleep.

Enjoy your amp it sounds like you've got a good one

I havn't done any projects for a long time. Last week I met an 85 year old gent that used to repair organs for a local music store. He gave me a box of nos tubes and said he would call when he gets his garage cleaned out.(I smell a gold mine)

[dorian]

olive2sing -- Thanks for the kind words. I use this amp fairly regularly now. With some tweaking of the gain, it can be set for a really touch-sensitive breakup, going from pretty clean to metallic buzz just by hitting the strings harder. It is certainly more touch sensitive than my Orange Tiny Terror.

I might change the 50 mfd capacitor C4 for a 5 mfd, but I'll probably leave the rest alone. You will see silly things on the schematic such as two parallel resistors or capacitors that can certainly be replaced with one of the right value. I have left them this way because some are a bit hard to reach now.

For the VVR, I drilled a hole in the front panel between the gain and master volume. It was a very tight fit, but it worked out, and I like the VVR. I am a bit surprised that it doesn't affect the tone so much, even when dropping the voltage by a factor of five or so.

Finally, I do recommend the bypass around the tone circuit. The tone circuit drops the level a lot, and for serious overdrive you want it out of the way.

Good luck with your NOS tubes and with the other stuff in your friend's garage. Since he repaired organs and not televisions, perhaps there will be enough gold in there to start a cool project.